“This incident is a heartbreaking tragedy that defies understanding,” Mundelein Mayor Steve Lentz said at a news conference, CNN reported. “I am asking the Chicagoland area, please pray for us. Pray, first of all, for the family that has been devastated by this.”

The girls lived in the home with their mother, who was not home at the time. The TV station talked with neighbors who said the girls seemed to be close to each other.

“Seemingly close. They would always be together. The older one would be taking care of the younger one, and vice versa,” Mary Ann Gryder told WLS.

The 14-year-old’s mother was in court with her daughter Wednesday. The 14-year-old was visibly upset, although she was described her as “calm and composed” too. WLS reported the girl told the court she woke up and stayed in bed about 15 minutes thinking about what she was going to do. She then said she went downstairs and got the knife and then went to her half-sister’s bedroom and began stabbing her.

The medical examiner testified that the 11-year-old had defensive wounds on her hands and arms where she tried to grab the knife.

The 14-year-old had taken a shower before calling the police, WLS reported.

Colleen Cicchetti, a Lurie Children’s Hospital psychiatrist, told WLS, “It was probably pure rage. We don’t know for sure but for anyone, even a stranger, to hurt someone that repeatedly is unusual. I don’t think we can know all [of] what drove it.”